Distributed temperature sensing basedon slope-assisted Brillouin opticalcorrelation-domain reflectometry
with over 10 km measurement range
H. Lee✉, K. Noda, Y. Mizuno and K. Nakamura
The authors demonstrate distributed temperature sensing based on
slope-assisted Brillouin optical correlation-domain reﬂectometry(BOCDR) with a long measurement range of >10 km. They ﬁnd thatto achieve such a long-range measurement, a delay line in a reference
path needs to be at least four times longer than the sensing ﬁbre. In
addition, they show that the use of such a long delay line induces
forward-propagating Brillouin-scattered light in the reference path,
which deteriorates the signal-to-noise ratio (SNR) of the systemand should be suppressed. Finally, by exploiting a beyond-nominalresolution effect of the slope-assisted BOCDR, they detect a
3-m-long heated section in a 13-km-long silica ﬁbre in a distributedmanner; the reason for its low SNR is also discussed.

Introduction: Brillouin scattering in optical ﬁbres has been widely used
to develop strain and temperature sensors because of its distributed
measurement capability. Several spatially resolving techniques –time-, frequency-, and correlation-domain techniques – have been
implemented [1–6]. Among them, Brillouin optical correlation-domain
reﬂectometry (BOCDR) is the only technique that has unique advantages such as single-end accessibility, random accessibility, high-spatial
resolution, and cost efﬁciency [5, 7]. One of the general disadvantagesof BOCDR is, however, its long measurement time caused by frequency
sweeping for the acquisition of Brillouin gain spectra (BGSs). This
shortcoming has now been mitigated by two types of frequencysweeping-free conﬁgurations: phase-detected BOCDR [8] and slopeassisted BOCDR [9]; we focus on the latter conﬁguration in this study.
Slope-assisted BOCDR has been proved to have unique features[10–12]; one of them is its beyond-nominal-resolution effect, withwhich a strained or heated section with a length shorter than nominal
spatial resolution can be detected [10, 12]. For instance, a 2-mm-longstrained section of a 2-m-long silica ﬁbre was successfully detected
when the nominal spatial resolution was 0.11 m [12]. Meanwhile,almost all the previously reported demonstrations of BOCDR
had measurement ranges shorter than ∼1.5 km [13] (only one exception
is referred to in the conclusion). If we exploit the beyondnominal-resolution effect, it may be feasible to detect a metre-order-longstrained/heated section along a long ﬁbre under test (FUT) even without
the use of special conﬁgurations for extending measurement range, suchas temporal gating [13] and double modulation schemes [14].
In this work, the operation of slope-assisted BOCDR with a longmeasurement range of >10 km is demonstrated. First, we show that adelay line in a reference path should be at least four times longer than
the sensing ﬁbre to achieve such a long measurement range. We also
show that the use of such a long delay line inevitably accompanies
forward-propagating Brillouin-scattered light in the reference path,
which leads to a deteriorated signal-to-noise ratio (SNR) and thus
should be ﬁltered out. Then, based on the beyond-nominal-resolution
effect, we demonstrate the detection of a 3-m-long heated section in a
13-km-long FUT. We also discuss the reason for the low SNR of thelong-range measurement.

is much shorter than the nominal spatial resolution can be detected(beyond-nominal resolution effect) [10, 12].
Setup: The experimental setup of slope-assisted BOCDR with a longmeasurement range is depicted in Fig. 1a. The output from a frequencymodulated laser (linewidth: ∼1 MHz) was divided into pump light andreference light. The pump light was ampliﬁed to 10 dBm using anerbium-doped ﬁbre ampliﬁer (EDFA) and injected into a 13-km-long
FUT (see below for details). The polarisation state was scrambled.
Here, it is notable that the incident power should not be too high,because the Brillouin threshold power of a long FUT is generally low
and, even when the frequency is modulated, most of the high-powerincident light does not reach the end of the FUT by stimulated
Brillouin scattering [15]. What is also unique to long-range measurement is the structure of the reference path; unlike in standard
BOCDR, a long delay line and an optical narrowband-pass ﬁlter(3-dB bandwidth: ∼10 GHz) were employed. One of the new ﬁndings
is that the length of the delay line should be at least four times
longer than the FUT. In this experiment, a 60-km-long delay line was
employed; otherwise, the ﬁrst-order correlation peak cannot bescanned along the entire length of the FUT (the use of a higher-ordercorrelation peak requires an even longer delay line, which is not ideal
considering its cost and propagation loss). Another ﬁnding is that,when the delay line is extremely long, in addition to the carrier atlaser frequency, forward-propagating Brillouin-scattered light, both
Stokes and anti-Stokes, is induced and appears as sidebands in the
optical spectrum. This results from the back reﬂection of spontaneous
Brillouin scattered components etc. These unintended Brillouin components in the reference path cause considerable noise in the ﬁnal
system output if they interfere with the Rayleigh-scattered light fromthe FUT. Therefore, the optical ﬁlter was newly employed in the reference path to suppress the Brillouin components. The Brillouin-scattered
light was ampliﬁed and heterodyned with the reference light. Then it
was converted into an electrical signal using a photodetector (PD); its
spectral power at 10.87 GHz (on the BGS slope) was output using azero-span mode of an electrical spectrum analyser (ESA) and monitored
using an oscilloscope (OSC). The video bandwidth and resolution bandwidth of the ESA were 10 kHz and 10 MHz, respectively. Averaging
was performed 128 times on the OSC.
The structure of the FUT is depicted in Fig. 1b. A 13-km-long silica
single-mode ﬁbre (BFS = 10.88 GHz) was employed as a FUT, and a considerable bending loss was artiﬁcially applied near its end to suppress theFresnel reﬂection. A 3-m-long section (11 km far from the proximal end)
was heated from room temperature (23°C) to 70°C. The modulation
amplitude was 5 GHz, and the modulation frequency was swept from
4.51 to 7.07 kHz, corresponding to the measurement range of 14.5 km
and the nominal spatial resolution from 26.7 to 41.9 m [5, 7]. Note thatthe variation of the resolution according to the measurement position is
striking when the FUT is extremely long.
pump
1
laser

2

PSCR
EDFA
long
delay

reference

FUT

3

EDFA

stokes
PD

ESA

OSC

optical filter

Principle: BOCDR resolves to sense positions using a correlation
peak generated in a FUT by sinusoidally modulating the frequency ofthe laser output [5, 7]. By sweeping the modulation frequency, thelocation of the correlation peak can be scanned along the FUT for distributed measurement. The nominal spatial resolution and measurement
range are determined by the modulation frequency and amplitude [5, 7].
In standard BOCDR, the information on strain and temperature
at a certain sensing position is determined from a peak frequency(i.e. Brillouin frequency shift (BFS)) after the acquisition of the whole
BGS, which requires relatively long measurement time. In contrast, in
slope-assisted BOCDR [9–12], the information on strain etc. isderived not from the BFS itself but from the spectral power at a constantfrequency on the BGS slope, enabling high-speed measurement. The
ﬁnal output of the system is a power-change distribution along a FUT,
which is not completely identical to the BFS distribution [9]. By
making the most of this discrepancy, a strained/heated section whichaheatedFUT~3 m~11 km~2 km

bFig. 1 Schematic setup
a Setup of slope-assisted BOCDR with a long measurement range. EDFA:
erbium-doped ﬁbre ampliﬁer, ESA: electrical spectrum analyser, FUT: ﬁbre under
test, OSC: oscilloscope, PD: photodetector, PSCR: polarisation scrambler
b Structure of FUT

Experimental results: The normalised power-change distributions near
the heated section (23, 50, and 70°C) are shown in Fig. 2a. Clear peaks

ELECTRONICS LETTERS 7th March 2019 Vol. 55 No. 5 pp. 276–278

were observed at the heated section. The shape of the waveforms at theheated section is not rectangular, which is valid considering the natureof slope-assisted BOCDR [9]. It is also valid that the peaks were broadened because the nominal spatial resolution at this position is 28.2 m.
The SNR was not high in this long-range measurement, because(i) the incident power was much lower than those of shorter-range
measurement, (ii) the weak incident power was reduced further during
propagation of the long FUT, (iii) the measurement position is far
beyond the coherence length of the laser (∼100 m) [16], and (iv) the
aforementioned noise caused by the Brillouin components in the reference path cannot be completely suppressed. Subsequently, we plotted
the maximal power-change at the heated section as a function oftemperature (Fig. 2b). The dependence was almost linear, indicating
that, although the SNR was low, a 3-m-long heated section was properly
detected at a >10 km distant position.

Acknowledgments: This work was supported by JSPS KAKENHI
Grant Numbers 17H04930 and 17J07226 and by a research grantfrom the Fujikura Foundation. The authors are indebted to Prof.
Hiroyuki Uenohara, Tokyo Institute of Technology, for providing thelong silica ﬁbres used in the experiment.
© The Institution of Engineering and Technology 2019
Submitted: 5 December 2018 E-ﬁrst: 24 January 2019
doi: 10.1049/el.2018.7998
One or more of the Figures in this Letter are available in colour online.
H. Lee, K. Noda, Y. Mizuno and K. Nakamura (Institute of Innovative
Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan)✉ E-mail: hylee@sonic.pi.titech.ac.jp

1.8
70°C

1.2

50°C
0.6

23°C

0
–0.6
10.7

normalised power-charge,
arb.unit

normalised power-charge,
arb.unit

References

10.9
position, km
a

11.1

11.3

1.2

0.8

0.4

0
20

35
50
65
temperature, °Cb80Fig. 2 Measured resultsa Power-change distributions near 3-m-long heated section
b Maximal power-change plotted as a function of temperature. Dotted line is alinear ﬁt

Conclusion: We demonstrated the operation of slope-assisted BOCDR
with a >10 km measurement range. The experimental setup needed tobe modiﬁed from a standard short-range setup. We found that a delayline of at least four times the length of the FUT should be inserted in
the reference path. We also showed that the long delay line induces
forward-propagating Brillouin-scattered light in the reference path andthat it should be ﬁltered out to avoid the deterioration of the SNR.
Finally, based on the beyond-nominal-resolution effect, a 3-m-longheated section in a 13-km-long FUT was detected. We discussed four
reasons why the SNR was relatively low in this measurement.
Although there is a report that BOCDR with such a long measurement
range properly operated [17], the temporal gating scheme was employed
to extend the measurement range [13] and, in that case, a long delay line
did not need to be used. Unlike this report, our study is useful on the
point that the inﬂuences of the long delay line on the performanceof BOCDR have been investigated for the ﬁrst time to the best of ourknowledge. By additionally employing the temporal gating scheme[13] and/or the double modulation scheme [14], we may be able toextend the measurement range further while maintaining theshort-hot-spot detectability.

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